Preparation of heterogeneous catalysts with active ferrous centers is of great significance for industrial and environmental catalytic processes. Nanostructured carbon materials, which possess free flowing π electrons, can coordinate with transition metals, provide a confinement environment for catalysis, and act as potential supports or ligands to construct analogous complexes. However, designing such catalysts using NCM is still seldom studied to date. Herein, we synthesized a sandwich structured ternary complex via the coordination of Fe-loaded humic acid (HA) with C=C bonds in the aromatic rings of carbon nanotubes (CNTs), in which the O/N–Fe–C interface configuration provides the confinement environment for the ferrous sites. Their research has been published in Nanoscale, 2015, 7, 2651-2658.

The team utilize Fe-loaded humic acid (HA) as a ligand to interact with CNTs and synthesize a ternary sandwich-type HA/Fe-CNT complex, which can confine the active ferrous centers at the interface between HA and CNTs. HA molecules possess rich oxygen/nitrogen-containing functional groups in the aromatic rings; thus, they can interact with CNTs via π–π stacking and provide coordination sites for iron cations as well. The synchrotron radiation X-ray absorption spectroscopy (XAS) studies reveal that the distorted octahedrally or tetrahedrally coordinated geometry contribute to the stabilization of ferrous sites. The quantitative analysis of XPS suggests that about 44.9% of Fe exists as Fe(II) species in the HA/Fe-CNT complex. The extended X-ray absorption fine structure (EXAFS) oscillations of Fe K-edge revealed the first shell of HA/Fe-CNT yields a Fe–O bond distance of 1.93 Å and Fe–C of 2.09 Å; moreover, the coordination numbers are 3.88 for Fe–O ( from two H₂O molecule and one carboxylate group) and 2.02 for Fe–C, which correspond with the six-coordinated Fe model with four Fe–O and two Fe–C coordination. The C K-edge XANES spectra, a strong hybridization between CNT C π* and Fe 3d orbitals occurs, which is beneficial for charge redistribution between them. The density functional theory (DFT) calculation shows that the ternary system (HA/Fe-CNT) possesses higher chemical stability than binary systems (CNT-Fe or HA-Fe). The HA/Fe-CNT complex shows high catalytic activities for the activation of O₂ and H₂O₂, which can be applied in the oxidative degradation of phenol red (PR) and bisphenol A (BPA) in aqueous media. 

Synchrotron radiation techniques provide crucial insights into the mechanism of O₂ activation that the octahedrally or tetrahedrally coordinated geometry at Fe center and the strong hybridization between CNT C p* and Fe 3d orbitals induces the charge discretization of the aromatic rings on CNTs, which facilitates O₂ adsorption and electron transfer from carbon to O₂, thus enhances O₂ activation. The reviewer evaluated that this work is a highly significant advance that is able to expand research into the carbon nanomaterials and other transitional metals. Moreover, HA is a naturally occurring organic substance; thus, the complex HA/Fe-CNTs can be obtained on a large scale using facile, economical, and green strategies.